1. Field of the Invention
The present invention relates to a charger for a printer, facsimile apparatus or similar electrophotographic image forming apparatus and, more particularly, to a charger capable of charging a desired object uniformly.
2. Description of the Prior Art
In an electrophotographic process, a photoconductive element or similar image holding member has the surface thereof often charged by a corona discharger. A corona discharger is capable of charging the surface of the image holding member to a preselected potential uniformly. However, generation of ozone is the problem with a corona discharger. To reduce ozone, there has been proposed a charger using a charging member in the form of a brush or roller and holding it in contact with a desired object while charging the object. The brush type charger has a laminate structure made up of an insulative substrate, a conductive layer formed on the substrate to be applied with a voltage, and a fur brush implemented by low resistance fibers. The brush may comprise a rotatable brush roller or a flat brush.
It has been customary with the brush type charger to apply only a DC voltage thereto. This brings about a problem that the charge potential deposited by the charger is susceptible to environment. Specifically, when temperature and humidity are low, the resistance of the brush increases to, in turn, lower the charge potential; in a hot and humid environment, the resistance of the brush decreases to, in turn, raise the charge potential. Particularly, when temperature and humidity are low, a streamer discharger occurs and causes white spots to appear in a printed mesh image.
In light of the above, Japanese Patent Laid-Open Publication No. 1-267667 issued on Oct. 25, 1989 teaches a charger capable of charging the surface of an image holding member uniformly without regard to changes in environment. This charger is implemented as a roller, brush or similar charging member and applies to the image holding member a voltage having a DC component and an AC component superimposed on each other. The AC voltage has a peak-to-peak voltage more than twice as high as a charge start voltage. Why this kind of scheme sets up uniform charging is, presumably, that the AC voltage transfers a charge not only from the charging member to the image holding member but also from the latter to the former; the reverse transfer from the latter to the former renders the charge uniform even when the charge on the image holding member is locally excessive.
The superimposed DC-AC scheme stated above is advantageously applicable to ordinary characters and lines to be printed. However, when it comes to an apparatus of the kind printing mesh images including halftone images, the dot size becomes irregular and, therefore, moire becomes conspicuous. In the worst case, white vertical stripes appear in the resulting image. Such an occurrence is particularly conspicuous due to the AC voltage applied to the charging member.